Cab signalling system for railroads



Jan. 1956 c. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS 3 nventor l Gttorneg Jan. 17, 1956 c. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS 6 Sheets-Sheet 2 Anz Bnventor l Cttorneg FIG. f3.

Jan. 17, 1956 c. F. STAFFORD 2,731,550

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GELE/B .2.53m mom Zunozo (Ittorneg C. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS Jan. 17, 1956 6 Sheets-Sheet 6 Filed June 50, 1950 |I||S z l1| lm zen 5&28 @ZES CCCCCCCCCCCC ccccccccccccm United States Patent O CAB SlGNALLlNG SYSTEM FOR RAILRADS Chester Friend Stafford, Rochester, N. Y., assigner to General Raiiway Signal Company, Rochester, N. Y.

Application .lune 30, 1950, Serial No. 171,423

Claims. (Cl. 246-63) This invention relates to a railway signalling system and more particularly to a signalling system providing for the control of train cab signals as Well as the usual Wayside signals.

To control cab signals on a train, electrical pulses occurring at distinctive rates are generally applied to the track rails. rl'he code rates most commonly used are 75, 120, and 180 pulses per minute although other suitable rates could also be used. Train-carried apparatus is then employed to detect the presence of the pulses in the track rails and also to decode the information contained therein to operate the cab signals. Because the characteristics of the code detection and decoding means used in most prior cab signalling systems are such that the inductive transfer of energy from the rails to the train cannot satisfactorily take place when only direct-current pulses are used, the electrical pulses applied to the rails are required to be or alternating current or at least contain a component of alternating current. Por this reason, many signalling systems employing direct current coded track circuits cannot be readily used for cab signalling, because the necessary additions to provide alternating-current pulses to the rails entail considerable expense.

The present invention, however, provides an organization in which the train-carried apparatus is properly responsive to trackway apparatus employing direct-current coded impulses. Suitable decoding means is used to obtain positive operation of the cab signals with these directcurrent pulses, but at the same time this decoding means is constructed to be unresponsive to foreign currents. The expense entailed, therefore, in adding cab signalling to an existing direct-current coded track signalling systern has been considerably reduced. Also, signalling systems using line Wire control rather than coded track may also readily be adapted for cab signalling by means of this invention, since the existing direct current track circuits of such a system can readily be converted to direct current coded track circuits.

ln addition, the cab signalling apparatus of this invention is operable over territory in which the code pulses applied to the track consist of rectified but unltered alternating current. This track current may be either fullwave or half-Wave rectified alternating current of the usual commercial power frequency, such as 6() cycles per second for example. Despite the ability of the system to operate properly in regions where rectified alternating track current is used, the apparatus does not respond to stray alternating track currents even when such stray currents have a frequency equal to that of the source of the rectified track current. Various other safety features are also incorporated into this cab signalling organization to prevent a false signal aspect from being displayed under certain conditions.

Other objects, purposes, and characteristic features of this invention will be in part obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference will be made to the accompanying drawings in which like reference characters-designate corresponding parts throughout the several views, and in which:

Fig. l illustrates a line wire signalling system using direct current coded pulses in the trackway for cooperating with the code receiving and decoding apparatus of this invention;

Fig. 2 similarly illustrates a direct current coded track signalling system for cooperating with the code receiving and decoding apparatus of this invention;

Figs. 2A and 2B respectively show how the code applying apparatus of Fig. 2 can be modied to provide track code pulses of half-wave or full-wave rectified alternating current; y

Fig. 3 shows the preferred form of the code receiving and decoding apparatus using a balanced amplifier circuit;

Fig. 4 shows an alternative form of code receiving and decoding apparatus using thyratron output tubes;

Fig. 5 illsutrates still another form of code receiving and decoding apparatus in which an Eccles-Jordan trigger circuit is used; and

Figs. 6, 7, and 8 are waveform diagrams for use in connection with the discussion of one possible theory of operation of the code receiving and decoding apparatus as it responds respectively to direct-current, half-Wave alternating current, and full-wave alternating current coded track circuits.

The parts and circuits of this invention are shown diagrammatically and conventional illustrations are used to simplify the drawings and the explanations. The drawings have been made to make it easy to understand the principals and manner or operation rather than to show the specific construction and arrangement of parts that would be used in practice. The relays and their contacts are shown in a conventional way. Symbols are used to indicate connections to the terminals of a battery or other source of electric power instead of showing all of the wiring connections to these terminals. The symbols (l-) and indicate the positive and negative terminals, respectively, of a suitable source of direct-current of relatively low voltage. The symbol (B+) and the symbol for ground, indicate connections to the positive and negative terminals, respectively, of another direct-current source of higher potential suitable for the plate-cathode voltages of the electron tubes used.

Tmckway apparatus of Fig. 1

Fig. l shows the way in which a block signalling system, wherein signal controls are transmitted over line wires, may be adapted for a cab signalling system. A stretch of track, divided in a plurality of track sections by insulated joints, is signalled for east-bound trame, i. e., from left to right, by signals such as signals 2 and 3 that govern the entrance of trains into track section 2T and 3T, respectively. A track battery is normally connected across the rails of each track section as shown, for example, by track battery TBZ connected through back contact 10 of relay 2AlJ to the rails of track section 2T. Each track battery has connected in series therewith a current-limiting resistor such as variable resistor il in series with track battery TBZ.

A retained neutral-polar line relay is included with the apparatus at each signal location. For example, relay 3H at the signal 3 location is a relay of this kind. The energization of this line relay at each signal location is dependent upon the energization of the track relay, or relays associated with the track section governed by the signal at such signal location. The polarity of such energization is determined by the energized or deenergized condition of the corresponding home relay H for the next signal in advance. Relay 3H can, therefore, be energized only when front contact 12 of track relay STR is closed,

ave-1,550'

Y and its polarity of energization depends upon the energization of the HV relay associated with the next track section in advance. Thus, relay ZH is energized with current passing ,from left to right through its windingsoy that its polarv contacts are actuated to their right-hand position when relay 3H is energized with its front contacts 13 and 14- closed. Conversely, the direction ofcurrent', is from right to leftthrough the windingV of relay ZH so that its polar contacts are actuated to their left-hand` positions when relay 3H is deenergized with its back contacts 13 and` 14 closed as shown. Because of the retained neutral characteristics of these H relays, their neutral contacts remain in an energized position when the polarity of the current through the relay winding. is reversed.

Assuming that there is a train TNl occupying track section 3T, the shunting eect produced across the railsV of this track section causes track relay STR to drop away.

Vcorresponding, to the slowV release time of relay3I-I caused by its retained neutral characteristics, this relay drops away thereby closing its back contacts. With back contact 16 of relay 3H closed, lamp R of signal 3 is illuminated causing this signal to display a red aspect. Assuming that there is no train occupying track section ZT, front contact 1S of relay ZTR isV closed and, with relay 3H dropped away, relay ZH is then energized with a polarity to actuate its contacts to their left-hand positions. With this relay ZH energized as described, a circuit is completed through front contact 1S' and left-hand Vcontact 19 to energize lamp Y of signal 2. This signal then displays a yellow aspect.

When a train such as train TN2 enters'track section 1T, the track relay associated with that track section drops away. Because the circuit organization at the signal location governing the entrance of trains into track section 1T is similar to that at the signal Z location, the dropping YThe opening` of front contact 12 of track relay STR then deenergizes the horne relay 3H. After a brief interval away of the track relay Yplaces energy on line wires Z1 to energize relay 1AP at the signal Z location. The closing of front Contact ZZ of relay lAPthen completes a circuit to energize code transmitters 1-75CT and 1-180CT. These code transmitters are preferably of the kind that intermittently close their contacts at specified rates when energized. Code transmitter 1-75CT is actuatedl at the rate of 75v times per minute, and code transmitter 1.-180CT is energized at the rate of 180 times per minute. Other rates may, of course, be used if desired; the 75` and 180 rates Vare proposed because they are commonly used in y coded signalling systems.

With both relays lAP and ZH at the signal Z location energized, a circuit is then intermittently completed through front contacts 23V and Z4- ofthese relays, respectively, and through front contact Z5 of the code transmitter 1-180CT to apply direct-current pulses to the rails of track section 1T. These pulses are then transmitted over track section 1T toward the next signal location to the rear of signal Z, but are shunted by train TNZ before reaching such signal. As will subsequently be shown, the cab receiving and decoding apparatus 31 associated with train TN2 responds to the coding of the track current by causing a green aspect to be displayed in the cabof train TN2 as long as pulses of the lSOYrate are applied to track section 1T.

When train TN2 moves eastward into track section ZT, track relay ZTR drops away and in doing so closes its back .contact Z6 thereby completing a circuit over line wires Z7 to energize relay ZAP. The energization of relay ZAP then opens the circuit through its back contact 10 torernove the steadily applied direct current that is normally applied to the rails of track section ZT. The'closing of front contact 2? of relay ZAP energizes both code-transmitters Z-CT and Z-tiCT. As already shown, relay 3H is deenergized because the occupancy of track section 3T by train TNl has caused the opening of front contact 12 or" relay ETR. Therefore, with back contact 2910i relay 3H closed, a circuit is then intermittently' completed throught frontv contact 10 ot? relay ZV, back contact Z9r of Y relay 3H, and front contact 30 of code transmitter Z-7 SCT. Because front contact 30 is.closed at the rate of 75 times per minute, direct-current pulses are applied to the rails of track section ZT at this rate of 75 times per minute. As will later also be shown, the cab receiving and decoding apparatus 31 causes a yellow.` aspect to be displayed in the cab of train TNZ as long as pulses of the 75 rate are applied tothe railsof track section ZT.

As trainTNZ crosses into track 'section 3T behind train TN1no code is received bythe cab receiving and decoding apparatus 31 because of the shunting eect on the rails of" track sectionST bytrain TN1. As a result, a red aspect is displayed'in the cab of train TN2.

As a train moves out of a track section into the track section in advance, the normal conditions previously described must quickly be restored. For example, as train TNZ moves eastward out of track section ZT into4 track section 3T (assuming trainTN1 to have vacated section 3T), the direct-current pulses applied to the rails of track section ZT when this tracksection 3T is occupied, can be transmitted all the way to the signal Z location because train TN2 no longer shunts track section 2T. The first of these transmitted pulses causes track relay ZTR, therefore to become energized and open its back contact Z6. The resulting removalofenergy from line wires Z7 then releases relayZAP and, since this relay ZAP is a quickreleasingrelay, its back contact'1'0 is quickly closed to apply steady direct current again to'the rails of track section ZT. In this way, the coding of track section ZT is quickly terminated and steady direct current is applied to track section ZT to hold track relay ZTR in an energized condition. Relay ZAP must have quick releasing characteristics so that the picking up of track relay ZTR will quickly cause back contact 10 ofV relay ZAP to close and place steady energy on the rails of track section ZT; This rapidL applicationv of steady energy to the rails is necessary'to hold relay ZTR picked up following its energization by a` code pulse for, unless relay ZTR can be held' in itsV pickedY up condition, relay ZAP will drop away and again causecode pulses to-be applied to track section ZT;

Trackway apparatus of Fig. 2

Fig. Z shows a typical block signalling system using direct current coded-.trackV circuits. Since the signal controlling organization is similar to that of prior kinds of codedy track systems, a description of the operation of the apparatusin-Fig. 2 will-be given only in general terms. In aa coded. track circuit signalling system of this kind,

Vpulsesof direct current are-applied to the eXit end of each track section. These on periods of the code occur at a selected rate that is indicative of traffic conditions in adjacent track sections; and, together with the off periods, they-comprise'- a driven code. At the `other end of each track section, a code-following track relay connected Yacross the rails isr energized by each on period of the means for approach lighting control and the like.

In- Fig. 2 the organization shown is arranged for a normal'traic direction of west to east, corresponding to left to right in this drawing; The'v driven code in each track section is transmitted, therefore, in theopposite direction, from east to west-withjthe inverse code transmitted in the normaldirection of` traffic. lnY this way, the entrance of al train into a track-section prevents theV driven code from being received at Vsuch entrance end. Because the track Vrelaya.tfthat1er1'd cannot'thenfbe energized, no inverse code can be transmitted to the exit end. The driven code will still, however, be transmitted toward the train and each pulse will cause a voltage to be induced in inductors 60 and 61. In a manner to be described, the induced voltages applied to the cab receiving and decoding apparatus 31 cause the proper signal aspect to be displayed in the cab of the train.

At each end of each track section shown in Fig. 2, a contact of a coding relay selectively connects either a winding of a track relay or a track battery across the rails of the associated track section. At the exit or right-hand end of track section T, for example, the closure of back contact of relay SCT connects the winding of track relay STRB across the rails of track section ST. When front contact 40 is closed, track battery STB is connected through current-limiting resistor 41 to the rails of track section 5T. At the entrance end of track section ST a similar organization is provided except that the circuit selection is made by a contact of relay SCTA. Relay SCTA is an inverse code transmitting relay at the exit end of track section ST which is momentarily picked up at the end of each driven code pulse.

At each signal location a contact of a track relay, in combination with a decoding transformer and a rectifying contact of the track relay cooperate to hold an H relay energized whenever code of any rate is received over the adjoining track section. At the signal 6 location, for eX- ample, the alternate closing of front and back contact 42 of track relay 6TRA causes relay 6H to be picked up.

The decoding apparatus of Fig. 2 also includes a conventional decoding unit such as the 180 rate decoding unit 39. Because of the characteristics of this decoding unit in combination with the decoding transformer 3S, the operation of the decoding organization is such that only if the code received over track section 6T is of the 180 rate, is relay 6D picked up.

The code transmitters at each signal location such as the code transmitters S-180CT and S-7SCT at the signal S location are continuously energized and close their front contacts 180 and 75 times per minute respectively. The

driven code transmitting relay CT at each signal location is energized at a rate depending upon whether the associated H relay is energized or deenergized. if relay 6H is dropped away, code transmitting relay SCT is energized through back contact 43 of relay 6H and through front contact 44 of code transmitter S-7SCT. Code transmitting relay SCT is then energized at the rate of 75 times per minute thereby completing a circuit through its back contact 40 to apply direct-current pulses at that rate to track section 5T. Similarly, if relay 6H is picked up and l its front contact 43 closed, direct-current pulses of the 1S() rate are applied to track section ST.

The apparatus at the entrance end to each track section includes an inverse code transmitting relay such as relay lSCTA at the signal S location. The operation of this resection is selected dependent upon the energization of the H relay at the exit end of that track section. The energization of that H relay is, in turn, dependent upon the occupancy of the next track section in advance. Assume the conditions shown in Fig. 2 Where a train TNS occupies track section 6T. Since no driven code pulse can then be received at the entrance end of this track section, both relays 6H and 6D drop away. The closing of back contact 43 of relay 6H then causes a 75 code to be transmitted to the signal S location.

Since there is no train occupying track section ST, the driven code transmitted from the signal 6 location is received at the signal 6 location and causes relay 5H to be picked up. Therefore, a code is transmitted westward from the signal S location. The cab receiving and decoding apparatus mounted on train TN4 responds to the ld code appearing in the rails of track section 4T by causing a green aspect to be displayed in the cab of train TN4.

When train TN4 moves eastward to track section 5T, driven code pulses of the 75 rate are still applied to the rails of track section ST and the cab receiving and decoding apparatus responds to this 75 code to display a yellow aspect in the cab. If train TN4 were to move into track section 6T, no code pulses could be received by the cab receiving and decoding apparatus and the cab signal would then display a red aspect. Thus, the block signalling sysern shown using direct current coded track circuits not only provides forv the control of the usual wayside signals but also places pulses upon the track rails which can operate the cab signals and thereby provide signal indications in the cab of a train informing the crew of existing traic conditions.

Train carried equipment Three forms of the code receiving and decoding apparatus 31 of Figs. l and 2 are shown as embodying this invention. in each embodiment, the train-carried apparatus includes inductive elements 60 and 61, commonly known as receivers, mounted adjacent the track rails. These series-connected receivers are so connected with respect to each other that the voltages induced therein are additive. capacitor 62 to constitute a parallel tuned circuit. This tuned circuit is then shunted in turn, by a resistance of a magnitude selected in accordance with the principles of the present invention. In the embodiments shown in Figs. 3 and 5 the shunting resistance consists of two equal series-connected resistors 63 and 64; whereas, in the embodiment illustrated in Fig. 4 the shunting resistance includes only a single resistor 67.

The receiver circuit is responsive to the coding of the track current to supply a distinctive output voltage at the beginning and end of each track code pulse. Briey, the distinctive voltage variations are caused by a shock eX- citation of the previously described tuned circuit at the beginning and end of each track code pulse. The shock excitations are caused by the voltage induced inV the receivers by each successive application and removal of track current. By properly damping these oscillations, only the desired distinctive voltage variations are retained to characteristically represent the beginning and end of each track code pulse. These distinctive output voltages are then suitably amplified and control the operation of an electromagnetic code following relay causing the contacts of this relay to be actuated to one position at the beginning of each track code pulse and to the opposite position at the end of each track code pulse. A decoding organization controlled by this electromagnetic relay governs the energization of a cab signal so that the particular aspect displayed at any time is dependent upon the rate of response of this code following relay and is, therefor-e, in turn dependent upon the rate of application or ccde pulses to the track rails.

in the preferred form of the code receiving and decoding means, shown in Fig. 3, the voltage appearing across the receivers is applied to both channels of a balance-d amplifier comprising tubes 69 and 71. The output of each channel of this rst amplifier is then applied to the corresponding channel of a second amplifier with the output of this latter amplifier controlling the operation of a code following relay CR. The code following relay CR in turn controls decoding apparatus comprising front and back contact repeating relay CRFP and CRBP, transformers 9S and 97, condenser 96, rectifier 98, and relay 180k. The relays CRBP and 180k control the cab signal having green, yellow, and red lamps G, Y and R, respectively.

In the embodiment shown in Fig. 5, the output of the The receivers 60 and 61 are shunted by a Yfor the remainder of that pulse period.

. 23ans-so coupledV withA the control grids of two triode tubes 132' and.. 133 includedinadual-triod'e trigger or iiip-fiop circuit.V Aco'deffollowingrel'ay CR is controlled by the response ofV these triode tubes and, in turn, controls the operation; of' the decodingapparatus so that the proper cab signal aspectisvdisplayed`,.as-shownin detail Fig. 3.

The third embodiment of the code receiving and decodingv apparatus shown. in Fig. 4 has but a single elec,- tron discharge tube 119 in the rst amplier stage. The output of. this amplifier is; transformer-coupled to the grids of two gaseous discharge or thyratron tubes 11tan-d 115. The. outputs of these, thyratrons then selectively control the. energization. of a code following relay CR withv this relay then controlling the decoding apparatus in the usuallway, asshown indetail in Fig. 3.

Operation relay SCT in Fig. 2, abruptly connects and disconnects the battery STB across thetrack rails of section 5T. The operation of such a coding Contact is graphically illustrated at line A of Fig. 6.

it will be appreciated that the track rails of the section Y Vvhave an inductive effect upon the rise and decay of the current in the rails, while there is also a certain amount of distributed capacity and also a considerable amount of ballast. leakage'under most circumstances. Thus,rat the beginningofeach on period, the track current rises over a period offtirne to a particular steady state value At the end of each on period of the code, the track current decays after a corresponding period to a zero value for the remainder of such. oi period. The particular rate of current rise or decay will vary from time to time as the `ballast conditions change, but they voltage of the battery .STB is made suflicient to provide a proper code pulse at. the entrance end to the track section under the rnost adverse conditions.. This current rise and decay in the track rails has been roughly indicated in the graphic illustration of line B in Fig. 6 merely to illustrate that the `track circuit conditions do have an eliect upon the rate of current. change at the leading and trailing edges of the 'code pulses.

As a train enters a track section and progresses towards the exit end thereof, a greater portion of the track rails and associated ballast is shunted out of the track circuit so that. the rate of rise and decay of track circuit current is increased as the train approaches the exit end of` the section. Thus, assuming that the code pulses are oi proper valueV for most adverse conditions (i. e., lou7 ballast and train. at entrance end of section), the'resulting code pulses during the passage of a train are always sufficient to operate the cab receiving and decoding apparatus 31;

With reference to the drawings, it will be readily apparent that the coils. 643 and 61 are inductively coupled tothe trackrails so that any current. change in the track .rails causes a change inthe magnetic tield surrounding the rails .to in turninduce a voltagey in the coils 6@ and 6l. Specifically, therise influx at the beginning of each'pulse in the. track rails induces a voltage in the windings 60 and 6l of. a particular polarity, while the decay of flux at the endor" each pulse inthe track rails induces a voltage in the windings 69. and 6l of the opposite polarity. e

Thesey induced voltages areof course very momentary;

but, since the. windings 66) andl are tuned to'resonance,

a rnomentaryvoltageshock excites this tuned circuit so as to produce transient oscillations therein.'l

. it wilLbe-appreciatedthat the receiver coils 60 and 61` small amount ot` distributed capacitancel which exists be"V tween the turnsV of'wire in the' receiver windingswliicl'rin'V eftect is in multiple with the windings-A causing tlem to act as parallel tuned circuits. If the condenserGZfwere not' connected' to the'coil's- 60 and 61; thislnat'ural resonant frequency of the reeeiverswould be relatively iii'glr;' and, upon the application of'each direct current code pulse to the track rails, a voltage would-beinduced intheY receivers which wouldl set up' a train of oscillationshaving a frequency corresponding to this relatively highnatural res` onant frequency of the windings. Similarly, at the*v end or each code pulse'of direct currentV inthe track rai-Is a voltage would likewise'be induced'inV the windings which would then set up a train of oscillations havingl afre Vqu'ency also correspondingv to the naturali resonantfreuency of the windings. Since the windings mustEv of necessity have a certa-in amount of resistance, thesetrains ot oscillations wouldL be dampedtosome extent as'graphically illustrated by the trains of. oscillations in line C of Fig. 6. Y

it will be noted that the ii'rsthalfl cycle of the-train of oscillations produced at therbeginni'ngof a pulse is. apositive halfcycle; whereas, the trainof oscillations atv the end of a pulse begins` with a negative half' cycle (assuming one set of connections). However, itt t-rainsfof oscillations such asindicated in linefC of Fig. 6'- are applied to the amplifiers 69 andv 15 ofFig. 3 thereisno fway in which the amplifiers can distinguish as to-whether any particular trainY belongs to the beginningV or' the end of a code pulse period Inotlier'words, the difference in the polarity ofthe first half cycle. of? any train of oscillations cannot be distinguished if the entireftrain of' oscil'- lations is applied to the amplifier. Y

Y in accordance with the present invention, it is` proposed to provide theresistors 63andf 6470 Fig.v Bftodamp the trains ofY oscillationsproducedin the receiverr windings tol substantially the critical value off damping;V Iff this-critical damping were applied-towhat may be termed the natural resonant frequency of the/receiverslli. and' 61g the resulting half wave transientY appearing across.the windings would be ot very short` duration. Since the embodiment of the present inventionA shownV in. Fig 3 preferably includesa codefollowing'relay, it is. desirable to have the damped transients appearing at the beginning and end of each pulse oi suicient duration and? amplitude to'efect the operation ot such codefollowingY relay. To provide such artransient, the condenser 62' isused to resonate the windings 6d and 6I to ay relatively low frequency, such as in theV order'ofv 5' to 20' cycles per second. ln one embodiment of Vtheinventionythe.` resonantV frequency wasy selected! as. 14 cycles perA secondv and the provision of resistors 63 and 640i a proper value for critical damping gave, entirelyr operative; results. In this instance, the voltage across the receiver appeared as graphically illustrated in line F of Fig. 6.

When the resistorsi63 andt64 are of such aV value as to give the conditionsof critical' damping, allr oscillations of each train of waves are. eliminated'. excepty the first half cycle of each train. This degree ot' damping. also decreases, to sortie` extent, the amplitude of each 'rsthalf cycle of a train of waves; For this-reason', itzhasbeen found that the resistance value of resistors'. 63 and 64' mayV advantageously be. increased to aivaluewhere near.l critical damping takes place (i. e. slightly above critical. damping). in one embodiment ofi the invention, it has: been found that the value of theV resistors 63l and. 64;:.1naysbe Y approximately twice their value for critically damped ond halty cycle: is. or; snella.v relatively small amplitude: that 9 itsr duration is of no particular consequence. However, it should be understood that the embodiment of the invention should not depart too far from the conditions of critical damping else the train of waves will not be reduced to a characteristic or distinctive half cycle for the beginning and end of each pulse. For example, assuming that the resistors 63 and 64 provide only slight damping, then the voltages appearing across the receivers would be successive trains of transient oscillations somewhat as shown in line E of Fig. 6. Obviously, the amplifying organization could not distinguish between these trains as to whether they related to the beginning or end of a pulse, and for this reason it is highly desirable to use substantially critical damping.

Over-damping will also produce but a single pulse at the beginning and end of each code pulse, but with such overdarnping the single pulse produced for each train waves will be of lower amplitude than for critical damping and also it will be of longer duration. Accordingly, the amount of over-damping should not be too great otherwise the amplitude of the resulting pulse would be below an operative value.

For the purpose of this disclosure, especially the form of Fig. 3, it should be understood that critical damping or near critical damping should be employed, it being preferable in most cases to use slightly less than critical damping for reasons above explained. In other words, the receiving circuit should be tuned to a relatively low frequency and then damped to substantially critical damping conditions.

If the condenser 62 should inadvertently become disconnected so as to not tune the receiver circuit at the relatively low frequency, then the circuit assumes its natural characteristic involving the distributed capacitance of the windings 6G and 61 and the connecting leads. With the resistors 63 and 64 so selected as to provide substantially critical damping for the receiver circuit tuned to the relatively low frequency by condenser 62, their resistance is of such a low value as to provide considerable over-damping for the natural resonant frequency of the receiver circuit without the condenser 62. Thus, accidental disconnection of the condenser 62 would result in a voltage pulse appearing across the receiver circuit of one polarity for the beginning of a pulse and of the opposite polarity for the end of such pulse, but, these pulses would be of such short duration and low amplitude as to be ineffective to cause normal operation of the cab signal equipment. Such over-damped voltage pulses in the receiver circuit are illustrated graphically in line D of Fig. 6.

The voltage appearing across the tuned receiving circuit comprising inductors 6i) and 61 and condenser 62 appears also across the series-connected resistors 63 and 64 shown in Fig. 3. Because resistors 63 and 64 have an equal value of resistance, one-half of the voltage across the tuned circuit appears across each of these resistors. Since the cathodes of tubes 69 and 71 are both connected through resistor 72 to the junction of resistors 63 and 64, connecting the grids of these tubes through respective grid resistors 63 and 70 to the remaining terminals respectively of these resistors causes the grid-cathode voltages of these tubes to be of opposite polarity. The grid voltages of the respective tubes 69 and 71 are not only of opposite polarity, but are both substantially of an amplitude equal to one-half the amplitude of the total voltage across resistors 63 and 64 because only relatively small voltage drops appear across grid resistors 68 and 70 and common cathode resistor 72. These grid input voltages have been graphically indicated at lines H and K of Fig. 6.

Tubes 69 and 71 are operated as class A, balanced amph'iiers. Both tubes are provided with self-bias by common cathode resistor 72. Resistor 72 has connected in parallel with it condenser 73 which provides sucient by-passing to avoid degeneration of the amplied signal.

tion of the circuit in the event that an intermittent shortcrcuit should occur between control grid and plate '-of either tube 69 or 71. A short circuit between plate and control grid of either of these tubes would cause the plate voltage to be applied across the parallel tuned circuit and would shock excite this tuned circuit with the result that spurious impulses would then occur. Because resistors 68 and 70 are of relatively large value of resistance, the shorting of plate to grid of either tube 69 or 71 instead causes most of the plate voltage to appear across these grid resistors, thereby preventing false impulses from being produced by the parallel tuned circuit.

The plate voltage of tubes 69 and 71 is supplied from the (B+) source through resistors 74 and 75, respectively. The plate output of each tube is fed through a blocking condenser to the control grid of an output amplitier tube. The plate output of tube 69 is applied through blocking condenser 76 to the control grid of tube 77; whereas, the plate output of tube 71 is applied through blocking condenser 78 to the control grid of output amplifier tube 79. The control grid of each of the output amplifier tubes 77 and 79 is connected through an associated grid leak resistor 8i) and 81, respectively, to a source of bias voltage. The cathodes of tubes 77 and '79 are both connected to ground.

Tubes 77 and 79 are both biased to cutoif by the fixed bias voltage (B-) applied through grid leak resistors 80 and 81. For either of these tubes to conduct, therefore, the input voltage applied to them from the preceding amplier stage must raise their total grid-cathode voltage above the cutoff level. Driving the grid of tube 69 positive and the grid of tube 71 negative at the beginning of each track code pulse causes tube 69 to conduct more plate current and at the same time decreases the plate current of tube 71. This condition causes the plate voltage of tube 69 to decrease vand the plate voltage of tube 71 simultaneously to increase. The resultant decrease of grid voltage of tube 77 can have no etect on the conduction of this tube because it is already biased to cutoi. The increase of voltage on the grid of tube 79, however, raises the grid-cathode voltage above the cutoii level so that this tube conducts and plate current then flows through the lower winding of code receiving relay CR actuating the Contact to a corresponding position. The opposite condition prevails at the end of each track code pulse for then the gridcathode voltage of tube 69 is decreased while the gridcathode voltage of tube 71 is increased. Tube 69 then conducts less plate current at the same time that the plate current of tube 71 is increased. As a result, the plate voltage of tube 69 and thus the grid-cathode voltage of tube 77 is raised. The plate voltage of tube 71, on the other hand, decreases and lowers the grid-cathode voltage of tube 79. Therefore, although tube 79 remains cut oir, tube 77 conducts and actuates relay CR to the opposite position. In this Way, each application of a direct-current code pulse to the rails causes relay CR to be actuated first to one position and then to the other. The contacts of this relay are thus actuated at a rate corresponding to the rate of application of code pulses to the track rails.

Each time that code receiving relay CR, included in each embodiment of the invention, is actuated to close its front contacts 93 and 91 (see Fig. 3), front repeater relay CRFP is energized through a circuit including these contacts and 91. If relay CR is actuated to its alternate positions at a rate corresponding to that at which direct-current pulses are applied to the track rails, front repeater relay CRF P will remain steadily picked up, despite its intermittent energization, because of its slow releasing characteristics. Front contacts 92 and 93 of relay CRFP will then be closed. Therefore, each closure of back contacts 90 and 91 of relay CR will energize back repeater relay CRBP. Since this relay is Vformer 95 is energized at the 180 rate.

avenue tentfenergizationz. Thisarrangement of front. andbacky repeater` relays of? the code receiving relay CRY ensures, bythe-picking up of relay'- CREP, thatrelay CRy is vactuallyenergized to oppositepositicns in response to thek code: pulses appearing on Vthe track relays andis not steadily held-'in oneposition.

Contact 94 of. relay CR, together with transformer 95', condenser'96, transformer 97, and rectier 98 provide a decoding circuit that enables relayiei to be picked up .whenever relay CRis energized at the-180 rate. The form ofdecoding circuit shown in Fig. 3-is typical of that! generally employed in codedV track circuitl signallingysystems. Briefly, each time that contact 94 of relay.4 CRl is energized toione position or the other, onehalf of the primary winding of transformerr is-energizedl through acircuit Vincluding this contactV 94iand also-l front contact- 991y of' back repeater relay CREE?.- The-secondaryl winding-of transformer g5 is shunted by f av series resonant. circuit including condenser 96 and the primary winding of transformer 97. This series resonant circuit is tuned to 180 cycles per minute with the result` Vthat the secondary winding of transformer 97 isfenergizedfonly when the primary winding cftrans- The alternating voltage appearing-across the secondary winding of transformer 97'is thenv rectified by full wave rectitier 9?; sothat. relay 18)R is energized with direct current. if relay CRv is not intermittently actuated to opposite positionsat the 180 rate in response to a 180 code applied to the track rails, relay ltiR will not be energized.

When relay CRY is energized in response to a 18() code,

' both. front contacts it) of relay CRBP and 161 of relay ISGR are closed. Lamp M32 of the cab signal is thenl energized tokdisplay a greenv aspect. If relay CR isenergi'zed'fatV the 75 rate, however, relay CREP is stili energized but relay 130B'. is dropped away. In that event, lamp 1&3 ofthe cab signal is then energized through front contactV E05 of relay CREP and back contact itil of relay 189B.. A yellow aspect is then displayed by the cab signal. if the contacts of relay CR remain steadily in one position or the other, both relays CRB? andv 189K are deenergized. A circuit is then completed through back contact li of relay CRBi3 and back contact 10i-of relay 1831i to energize lamp 195 of the cab siglnal. A red' aspect is then displayed by the cab signa Byy means of thecircuit organization shown in Fig.

3' a cab signal may be selectively energized to display a proper signalV aspect dependent upon the rateat which the CR relay is energized. Although the 180v and 75 rates have been chosen as indicative of clear and caution signal conditions, other code rates may, of course, be used as desired. By using additional code rates, a larger number of different signals aspects may be displayed by the cab signal.

When the cab signaln'ng equipment is used in territory where commercial alternating current distribution lines are adjacent the right of way, stray currents of the Vcommercial power frequency frequently appear in the track rails. These stray currentsV may induce corresponding voltages in the train-carried inductorsV 6ftV and 61. To ensure that these induced voltages will not adversely affect the output of the decoding means, a series inductance-capacitance circuit is connected between theV plate of each of tubes 69 and 71 to the positive voltage source. The plate of tube 69 is connected through condenser 82 and inductance 83 to the (B+) source; whereas, the plate of tube 71 is connected through condenser 84 andinductance S5 to this same source of positivepotential. The valuesof the inductance and capacitanceconnected from each tube plate to the (B+) source are such that resonance of Yeach series circuitV occurs atV theV frequency of the interfering source, to-

thereby. provideaatrap .circuit-for theusual commercial frequencies., A: low-impedance shunt-path then. exists;r

for these frequencies .and-.they arerthusprevented from being fedthrough couplingcapacitors176 and78 to.v thev grids of tubes 7 7 and 79. An additional function of these series resonant trap circuits willl presentlyV be described.

Several otherA advantages result'from the-fact that the embodiment. of the. invention shown in Fig. 3 employs two channelsrof amplification. One of these advantages is that intermittentl breaks in either the grid or plate c ircuitsv of either tube 69. or 71l will not Ycause the relayY to be actuated toopposite positions; it will instead remain energized in one position. Should the common cathode connectionof` tubes @and 7l intermittently break, pulses ofthesamepolarity will be applied to the gridsof bothI tubes '77 annif/'9; and, since the relay Ywindingsrare in opposition, the` relay will notbe erroneously operated. Also, since theV output amplifiertubes 77 and 79 are biased to cutol intermittent breaks in the connections; to these tubes also will not cause the relay toy bel actuated alternately to opposite positions.Y

Thefaboye description considers the operation of the system whenY direct current coded pulses are employed 'Y inthe trackrails, but it should be clearly understood that the system, operates just aswell when the code pulses are rectiiied alternating current either half-wave orfull- .vaye'rectication. Referringvto Fig. 2, the track code transmitting circuitsrnay each be modified to be supplied with rectied alternating current. This has been shown for the exit endof track section T in Fig. 2A where a half-.wave rectifier` 5R is shown as connected in series with the secondaryof a transformer supplied from a suit-V able source of alternatinggcurrent. Such source of alter- :eatingV current; may be the usualy commercial 60 cycle alternating pot-.ver line, ormay be any desired alternating current source Ywhich might be used for such av purpose. It is tobe understood that the exit end of each track section may be consideredA as thus modified, and also that each entrance, end; may likewise he modified if desired. Tghe change ofl theV transmitting circuits to include thehalfwave rectified currentY has only been shown in connectionwith the-leaving end of the track section 5T, since it is the pulses transmitted from that end to whichthe traincarried apparatus is responsive.

The; operation of Yeach coding contact, such as contact etici relay CT, operates at time spaced intervals tov produce Code pulses at the selected rates. VThis operation of Contact itl has been indicated in line A of Fig. 7 The code pulses applied to the track rails consist of a number of,y half-wavesof alternating current as indicated graphicallyy in line B. of Fig.V 7.V

Let us assume. that a train with vehicle-carried equipment-asshown in Eig; 3 entersthetrack section 5T. Then the coderpulsesoffrectied alternating current act on; the receivers 6i)V and 6 1,V to produce induced voltages which act onlthe, code receiving and decoding apparatusv in a way to give the sainel operation as describedl for the direct currentV code pulses.

More specifically, each half-cycle of the half-waveV rectied alternating current in each code pulsejproduces a complete alternating currentV component in the receivers 60 and; 61 corresponding to, the frequency of the alterhatingcurrent source, since the inductive relation of the receivers to the-track rails is the equivalent of a transformer. However, the base lineor average value of the alternating current component of the induced Voltage varies in a manner similar to the transient produced at the, beginning' of a direct current code pulse. rhis` is illustrated graphically in line C of Fig. 7 The end of the code pulse produces a negative, transient voltage which is wholly'freeofiy any alternating current component, as graphicallyshown in line C of Fig. 7'. Thus, the, over-al1 transient component; of; thevoltage induced in the receiyers by: ahalf-wave. re. 2.ti felY almaillg CUIIQIA. Gilde Y 13 pulse is very similar to the transient voltage induced by a direct current code pulse in the track rails (compare lines C of Fig. 7 with line G of Fig. 6).

The transient component of the induced voltage of the receiver circuit is damped by the resistors 63 and 64, the same as previously described for the transient present due to the direct current code pulses. Thus, this transient component provides a distinctive characteristic at the beginning and at the end of each code pulse to give the same operation of the code following relay CR as previously described. In this connection, the amplifier tubes 69 and 7i amplify the induced voltages of the receiver circuit to provide the desired amplitude of input for the control tubes 77 and 79. The trap circuit means, including inductances 83 and 85 together with condensers S2 and 84, is effective to remove most of the fundamental frequency of the alternating current component. In fact, the frequency of the predominant alternating current component remaining in the voltage applied to the grids of tubes 77 and 79 is in the order of the second harmonic of the harmonic of the fundamental alternating current supply, but this second harmonic is of a very small amplitude. This has been illustrated graphically in line D of Fig. 7.

The major voltage variation in the input to the tubes 77 and 79 is the positive and negative transients at the beginning and end of each code pulse, and these characteristic voltages cause the operation of the code following relay CR exactly the'same as previously described. The recurrent alternating current component existing between the positive and negative transients is of such small amplitude as to be ineffective to cause the tubes 77 and 79 to produce an output of suicient magnitude to actuate relay CR.

Furthermore, should this alternating-current component of the input tubes 77 and 79 be of any appreciable value for any reason, it cannot cause operation of the code following relay CR because the relay CR is preferably so constructed that its armature has suicient inertia or the relay has sufficient magnetic retardation as to be unable to follow the relatively high frequency of the second harmonic of any fundamental frequency that might be employed as a source for the trackway code pulses.

It may be also pointed out that the code transmitting circuits of the trackway apparatus as shown in Fig. 2 may be modified as shown in Fig. 2B to provide fullwave rectified alternating current for the trackway code pulses. This Fig. 2B shows a full-wave rectier SRR connected to the secondary of a suitable transformer having its primary supplied with alternating current from a suitable source. It is to be understood that the exit end of each track section may be thus modified, and in addition similar full-wave rectified alternating current may be supplied to the inverse code transmitting apparatus at the entrance end to each track section if desired.

The operation of each code transmitting contact, such as contact 40 of relay SCT, operates to apply code pulses as indicated in line A of Fig. 8. The opening and closing of this contact apply time spaced code pulses at the rate then selected by traic conditions. Each code pulse thus applied to the track rails consists of a number of adjacent half-waves of unidirectional current, as indicated graphically in line B of Fig. 8.

Assuming that the vehicle-carried apparatus of Fig. 3 is located on a train entering the track section 5T, then the code pulses act on the receivers 69 and 6i to induce therein voltages which give the same operation of the code following relay CR as described above in connection with direct current trackway code pulses or half-wave rectified alternating current code pulses.

The successive half cycles of alternating current in each code pulse produce a complete alternating current component in the voltages induced in the receiver windings. ln other words, the induced voltage has an alternating current componentA having a frequency in the order of the second harmonic of the fundamental frequency of the alternating current source supplying the trackway code pulses. Since this frequency is considerably higher than the frequency to which the receiving circuit is tuned, the receiving circuit discriminates t0 some extent against this frequency. Obviously, this relatively high frequency is higher than that induced as the result of the use of half-wave rectified code pulses, and for this reason a greater discrimination is effected by the tuned circuit so that the ripples illustrated in line C of Fig. 8 are shown considerably smaller than those shown in line C of Fig. 7.

It should also be noted that the base line or average value of the alternating component of the voltages induced in the receiver windings varies in a manner similar to the transients produced at the beginning and ending of a direct current code pulse in the track rails. This has been illustrated graphically in line C of Fig. 8. Also, the end of the code pulse produces a negative transient voltage which is wholly free of any alternating component, as graphically shown in line C of Fig. 7. Thus, a comparison of the voltages induced in the receivers for fullwave rectied code pulses with the transients induced as the result of direct current code pulses shows that the transient components-are substantially the same in both cases.

The voltages induced in the receivers are supplied to the input of control tubes 69 and 71. Since the alternating current component of such input voltage is substantially the second harmonic of the fundamental frequency, the trap circuits in this case do not materially affect the voltage but they are present to by-pass any of the fundamental frequency which might be erroneously present. In brief, with full-wave rectied track current code pulses, the improved discrimination of the tuned receiver circuit against the second harmonic component reduces the magnitude of that component to such an extent that it appears primarily as a ripple of the major transient voltage. The positive and negative voitage swings of the transient act on the control tubes to cause the operation of the code following relay CR exactly the same as previously described.

lvlodificatz'ons The two alternative forms of the code detecting apparatus shown in Figs. 4 and 5 both include the parallel tuned circuit comprising inductors 6i) and 61 and condenser 62. Also, this tuned circuit in these alternative forms is shunted by resistance to dampen the oscillations that have been described as being produced by code pulses in the track rails. As explained, this damping resistance includes but a single resistor in the embodiment shown in Fig. 4 but the magnitude of this single resistor is preferably chosen to be substantially equal to the sum of the magnitude of the resistors 63 and 64 of Figs. 3 and 4 so that approximately the same degree of damping is obtained.

The embodiment of the code detecting apparatus shown in Fig. 4 includes an amplifier tube` lit and two thyratron tubes l14 and 115. Because no voltage dividing arrangement is here provided by means of equal resistors shunting the parallel tuned circuit, substantially the full voltage appearing across the receivers is applied to the control grid of tube lit). Thus, when this apparatus is used in regions where direct current is employed in the track circuits, the grid voltage of tube lit? is of the general form shown in waveform G of Fig. 6. Similarly, waveform D of Fig. 7 and waveform C of Fig. 8 illustrate the general form of the input voltage to tube 11) when the code detecting apparatus is used in half-Wave and full-wave rectified track current territories respectively. Regardless of whether direct current or rectified alternating current is used in the track circuits, however, an essential fact is that distinctive voltage swings of oppo- Aof each track code pulse.

Resistor 111 connected between the cathode of tube 110 and ground provides a class A cathode bias for this tube. Resistor 111 is shunted by condenser'112 which is of sufcient magnitude to avoid degeneration of the input signal. VThe proper screen grid and platepotential of this tube are supplied from the (B+) source through screen-dropping resistor 113 and primary winding of transformer T1 respectively. Screen currents of the signal frequency are by-passed to the cathode by condenser 117 to minimize degenerative elects.

As the grid voltage applied to tube 11|) alternately causes the plate current of this tube to increase and de crease about its quiescent value, corresponding voltage pulsations appear across the secondary windings of transformer T1. For any polarity of voltage induced across the secondary winding of this transformer, corresponding voltages of opposite polarity are applied to the control grids of thyratron tubes 114 and 115. Thus, if at the beginning of a track code pulse, the grid of thyratron 114 is driven positive, the grid of thyratron'115 will be driven correspondingiy negative. rhe'n, at the end of that track code pulse, the conditions will be reversed with the grid of thyratron 114 being driven negative, and the grid of thyratron 115 positive.

The control grids of thyratrons 114 and 115 are both preferably biased to cutoi by their being connected through a grid resistor and transformer winding to the negative terminal of a source of bias potential such as battery 116. Grid resistors 11S and 119 act as decoupling resistors and also limit the grid current of each thyratron tube when it is in a conducting state.

ri`he plate of thyratron 114 is connected through a load resistor 124 and the upper winding of relay CR to the (B+) source. The plate of thyratron 115 is similarly connected through a load resistor 125 and the lower'windingV of relay CR to the (B+) source. A condenser 126 Vis connected from the plate of thyratron 111 to the plate of thyratron 112.

Assume that at the particular moment to be considered thyratron 115 is conducting and thyratron 114 is not conducting. The plate current of thyratron 115 through the lower winding of relay CR holds the armature of this relay steadily in Vone position. Assume now that the potential on the grid of tube 114 is increased at the same time that the potential on the grid of tube 115 is decreased. The decrease of potential on the grid of tube 115 can have no effect upon Vthe conducting status of this tube because grid control of a conducting gas tube cannot be practicably accomplished. if, however, the grid of thyratron 114 is driven sufciently positive with respect to its cathode so that the tiring potential of this tube is exceeded, thyratron 114 will conduct. The relatively large plate current through its load resistor and the winding of relay CR then causes its plate potential to decrease. Condenser 126 then discharges, and momentarily lowers the plate potential of thyratron 115 to a relatively low value. Although the negative pulse applied to the grid of tube 115 could not of itself effect the nonconduction of thyratron 115, the sudden decrease of the plate potential of this tube accompanied by the decrease of its grid potential does permit this tube to be cutoff. The nal result is then that thyratron 115 is cut ott when thyratron 114 is rendered conductive. With the energization of the respective windings of relay CR reversed, this relay is then actuated to its opposite position. As soon as thyratron 115V is cut ott, its plate potential rises and condenser 126 again charges thereby suddenly raising the plate potential of thyratron 114. This increase of plate potential of the conducting thyratron does not affect, to any marked eX- tent, the conduction of this tube.

The next pulse applied to the grid of thyratron 116 is of such polarity that it drives the grid ofV thyratron 114 negativeV and the grid of thyratron 115 positive. By

derived from these tubes. Ycathode resistor 134 which is shunted by condenser 135 p lo analogy with the previous description, this occurrence causes thyratron 115 to conduct again and the negativeV pulse appearing on the plate of tube- 114 along witlrthe 'decrease of its grid potential then cuts oi thyratrorr 114. Relay CR is then actuated once more to its opposite position. Thus, this relay CR is operated to one position and thenV the other at a rate corresponding to the rate of application ofv code pulses to theV track rails.

The embodiment of Fig. 5 includes two electron tubes 13) and 131 having their grids connected to opposite terminals of the parallel tuned circuit and their plates connected to opposite terminals of a primary Winding of transformer T2. Since the cathodes of these, tubes are both connected to the junction of resistors' 63 amd64 shunting the parallel tuned circuit, substantially one-half of the voltage appearing across the receivers 68 and 61 is ap-V YAs the grid of one of these tubes is driven more positive with respect to its cathode at the beginning of a trackV code pulse, the grid of the other tube is driven vcorrespondingly morev negative. At the end of each track code pulse,'the grid of the rst of these tubes is driven negatively and the other tube is then driven correspond ingly more positive.

Class A self-bias is preferably used with respect to tubes 13) and 131 in order that maximum gainrmay be This self-bias is providedvby to Vavoid degeneration of the input signal. If desired, however, other bias levels maybefernployed, and if a source of fixed bias potential is used the bias of these tubesrnay be adjusted to cutoff.

' The input voltages applied to tubes 130' and 131 causes one of these tubes to conduct more heavily and the other toconduct less at the beginning of each code pulse in the track rails. As a result, a distinctive voltage yariation or a particular polarity appears across the secondary A winding of transformer T2. At the end of each pulse in the track rails, the situation is reversed, and a distinctive voltage variation of opposite polarity then appears across this secondary winding of transformer T2. Thus, a distinctive voltage of one` polarity appears across this secondary winding atthe beginning of each code pulse, and

` a similar appearing voltage of the opposite polarity appears across this secondary winding at the end of each code pulse.

The secondary winding of transformer T2' Vhas a ruid-V tap connected to the negative terminal of a suitable source of bias potential such as battery V137. The remaining terminals of this transformer secondary winding are connected to the grids of tubes 132 and V133 through Vresistors 13S and 139, respectively. The plate of tube 132 is .connected through resistor to the control grid of tube 133. The plate of tube 133 is similarly connected through resistor 141 to thecontrol gridof tube 152. The plate of each of vthese tubes is also connected through a corresponding load resistor Vand a winding of relay CR to the(B--) source. The cathodes of both tubes 132 and 133 are connected to ground. Y

The circuit organization with respect to tubes 132 and 133 constitutes an Eccles-Jordan, trigger circuit. Al-

though shown as including triode tubes 132 and 133, other tube types'such as beam power tubes may be used'for this circuit if desired. As is common with such trigger circuits, normally only one or the other ofthe two tubes included therein can be conducting and the other tube is at that time necessarily cut off. If, for example, tube 132 is conducting, the decrease of its plate potential because of plate current flow through load resistor143 causes, because ofthe connection through resistor 140 to the grid of tube 133, a decrease of potential on the control grid of tube 133 sufficient to bias this tube beyond cutoff. With tube 133 nonconductive, yits high plate potential insures that the grid of tube 132 is at a suitably high potential. Actually, of course, `the controlgrid of ,tube 132 is maintained at the cathode potential by reason of grid current ow through resistors 141 and 142 and the lower winding of relay CR to the (B+) source.

Assume at the beginning of a track code pulse the polarity of the voltage swing appearing across the parallel tuned circuit is such as to make the upper terminal of the secondary winding of transformer T2 negative with respect to the bottom terminal of this winding. A negative pulse is then placed upon the control grid of tube 132 and a positive pulse at the same time is then applied to the control grid of tube 133. If the conduction status of this trigger circuit at this time is such that tube 132 is conducting and tube 133 is not conducting, the negative pulse applied to the grid of tube 132 will cause this tube to become cut off. The sudden reversal that then takes place causes tube 133 to conduct because the decrease of plate current of tube 132 causes an increase in the grid voltage of tube 133. Previously, the conduction of tube 132 caused the energizetion of the upper winding of relay CR. Now, however, with tube 132 cut 0E and tube 133 conducting, this upper winding is deenergized and the lower winding of relay CR is instead energized. Each beginning and end of a code pulse in a track rail produces a corresponding pulse across the secondary winding of transformer T2 with successive pulses of opposite polarity. Consequently, the conducting status of tube 132 and 133 continually shifts from one to the other at a rate corresponding to the rate at which code pulses are applied to the track. Therefore, relay CR is also actuated to alternate positions at corresponding rates.

In this embodiment of the invention as in the embodiment shown in Fig. 4, the energization of relay CR is steady between successive reversals of energization. Therefore, although relay CR has been shown as a polar magnetic stick relay, this relay may, if desirable, instead be a three position polar relay.

Another possibility in connection with Figs. 4 and 5 resides in the fact that the relay CR could be replaced with two neutral relays with their contacts so connected as to jointly govern the decoding apparatus illustrated in Fig. 3. These alternative types of relays have been mentioned merely to emphasize the fact that in these forms of Figs. 4 and 5 the plate current of one or the other of the output control tubes is always flowing, and the magnetic stick characteristics of the polar relay CR are not required.

For the sake of simplicity in the drawing no trap circuit for interfering frequencies has been shown in the output circuit of the amplifiers of Figs. 4 and 5. However, itv should be understood that a condenser and inductance, resonant to an interfering frequency, may be connected across the primary windings of the respective transformers T1 and T2 if desired for reasons discussed in connection with Fig. 3. It should be noted that all forms of this invention provide a composite organization in which trackway code pulses may be of direct current supplied from a battery or may be rectified alternating current of either the half or fullwave type. In any event, the use of these different types or unidirectional current provides that the vehicle-carried equipment responds distinctively at the beginning and end of each code pulse so that uniform operation of the code following relay is provided. This facilitates in the proper decoding of the rates employed.

The fact that the code pulse detecting means distinctively responds at the beginning and end of each trackway code pulse is particularly advantageous in a cab signalling system where safety is involved. This is be cause the failure of a particular tube or the breaking of a wire which might cause an intermittent connection cannot reproduce the effect provided by the trackWay code currents. For this reason, the present organization vis believed to be particularly advantageous over any similar system provided in the prior art.

The organization is further characterized by the feature that stray or undesired foreign voltages of alternating current are rendered ineective to cause` an erroneous indication in the cab signalling equipment.

Having thus described sJzferal forms of the present invention, it is desired to be understood that the various forms are selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume; and it is to be further understood that various modications, adaptations, and alterations may be applied to the specic form shown to meet the requirements of practice, without in any manner departing from the spirit or scope of the present invention.

What I claim is:

l. In a cab signalling system for coded track circuits having code pulses of unidirectional current applied to the track rails at different rates dependent upon traffic conditions, vehicle-carried equipment comprising, receiver coils positioned in inductive relation to the track rails, means including capacitance and resistance connected with said receiver coils for damping said coils substantially to the critical value for giving a unidirectional output voltage of one polarity when each code pulse is applied and of the opposite polarity when each code pulse is removed, relay means having two different operating windings, means including a vacuum tube amplier responsive to the output voltages of said receiver coils for supplying current to said windings of said relay means alternately as each code pulse is respectively applied and removed, and decoding means governed by said relay means for operating contacts differently in accordance with the rate of the code pulses existing in the track rails.

2. In a cab signalling system for use with a track circuit supplied with code pulses of unidirectional current applied at different selected rates in accordance with traic conditions, the combinations including vehiclecarried apparatus comprising, a two channel ampler capable of giving two distinctive outputs one for each channel, receiver coils positioned adjacent each of the rails and both connected in series to said two channels of said amplifier, circuit means including capacitance and resistance connected to said coils to tune said coils and to dampen the oscillations set up therein by the application and removal of said unidirectional pulses in the track to thereby cause one channel of said amplifier to give its distinctive output upon the application of each pulse in the track and cause the other channel of said amplifier to give its distinctive output upon the removal of each pulse from the track, relay means operable to two different conditions, circuit means connecting said relay means to said two channels of said amplifier for operation to its opposite conditions in response to said two distinctive outputs respectively, and decoding means controlled by said relay means to give dierent indications in accordance with the rates at which said relayA means is operated by said pulses in the track.

3. In a cab signalling system for use with a track,l

circuit supplied with code pulses of unidirectional current applied at different selected rates in accordance with trac conditions, the combination including vehicle-carried apparatus comprising, receiver coils located on the vehicle adjacent the track rails, capacitance connected across said coils for tuning them to resonate at a relatively low frequency below the usual commercial frequencies, a damping resistor connected across said coils for damping to substantially the critical value the transient oscillations set up therein by the beginning and end of each code pulse in the track rails, said transient oscillations being thereby of opposite polarity for the beginning and end of each code pulse respectively, amplier means connected to said coils for amplifying said transient oscillations, a code following two-position relay connected to said amplier and operated alternately between its said two positions in accordance With the successively opposite polarity of outputs of said amplier, and decoding means selectively controlled by said code following relay to give different indications in ac-` across said coils for tuning them to resonate at a relatively low frequency below the usual commercial frequencies, a damping resistor connected across said coils Vfor damping to substantially the critical value the transient oscillations set up therein bythe beginning and end of each code pulse in the track rails, said transient oscillations being thereby of opposite polarity respectively Vfor the beginning and end of each code pulse, amplifier means connected to said coils for amplifying said transint oscillations, a code following two-position relay connected to said amplifier and operated alternately between its said two positions in accordance with the successively opposite polarity of outputs of said amplifier, a trap circut means of the resonated type tuned tothe usual commercial frequencies, said trap circuitV means being associated with said amplifier for removing the elfect of any stray voltages ofsaid commercial frequencies from the output of said amplifier -to said code following relay, and decoding means selectively controlled by said code following relay to give different indications in accordance with its rate of operation in response to Yapparatus comprising, pickup circuit means including receiver coils adjacent the track and inductively coupled thereto and also including a capacitance for tuning said pickup circuit means to a relatively Vlow frequency, a damping resistor means connected lto said pickup circuit means to cause it to give one distinctive output at the beginning of each current pulse in the track and to give a differently distinctive output at the end of each current pulse in the track, amplifier means connected to said pickup circuit means and including a plurality of vacuum tubes for giving correspondingly distinctive outputs as provided by said pickup circuit means, relay means operable to two diierent conditions,` circuit means connected between said amplifier and said relay means for Vsupplying one distinctive output to said relay for opmeans controlled by said relay means to give different indications in accordance with the rates at which saidV relay means is operated to its opposite conditions in respouse to said unidirectional current pulsesV in the track.

6. A cab signalling system for railroads comprising,V a plurality of track circuits and means for applying code pulses of unidirectional current to said track circuits, Vehicle-carried apparatus including receiver coils mounted in inductive relationship to the rails of said track circuit, capacitance connected with said coils to provide a resonant circuit tuned to a relatively low frequency below the usual commercial power frequencies, two'equal damping resistors shunting said receiver coils, kamplifying meansrincluding two channels of amplification and comprising, two input amplifier electron tubes biased Vabove cutoff having their control grids connected to opposite terminals of said receiverV coils and both cathodes connected to the junction of said resistors, a plate-cathode circuit'forl each of said input amplier ytubes including a load resistor, coupling circuit means for applying the output voltage across each of said load re-v sistors between grid and cathode of corresponding relay control tubes'both biased to cutoff, relay means operable to two different conditions and having two windings, a plate-cathode circuit for each of said relay control tubes including a winding of said relay means and a'source of direct-current potential,v and decoding means selectively operated by said relay means according to the coding rate of the track current.

7. A cab signalling system for railroads comprising, a plurality of track circuits and means for applying code pulses of unidirectional current to said track circuits,

' vehicle-carried apparatus including receiverV coils mounted in inductive relationship to the rails of said track, capacitance connected with said coils to provide a resonant circuit for a relatively low frequency, resistance `connected across said coils to dampen the electrical oscillations Vin said coils so that a distinctive voltage output of. one polarity is provided for each application of a code pulse to said rails and another distinctive voltage output of opposite polarity is provided by each'removal of a code pulse from saidrrails, amplifying means including twoY channels of amplification connected'to said coils, relay means of the polarized type voperable to two differentv conditions in accordance with its polarity Vof energization and including two windings each controlled by a respective .channel of said amplifier, whereby said distinctive output voltage of one polarity causes said relayV Y means to be operated to one condition and said distinctive voltage output of the opposite polarity causes said Y relay means to be operated to the opposite condition, and Vdecoding means governed by said relay means and selectively operated according to the coding rate of the track current.

8. A cab signalling system comprising, a plurality of coded track circuits having pulses of Vhalf-wave rectied alternating current applied to the track rails, traincarried apparatus including receiver coils mounted in inductive relationship to said rails, capacitance connected with said coil to tune said coils to a relatively low frequency, resistance connected with said coils and capacitance to dampen the oscillations in said coils and capacitance to produce a distinctive output voltage of one polarity at the beginning of each code pulse and yanother distinctive output voltage of opposite polarity at tinctive output voltages, and decoding means governedby said relay means'and selectively operated according to the coding rate of the pulses applied to the track rails.

9. In a cab signalling system for railroads, a plurality of coded track' circuits and means for applying pulses of unidirectional current to the rails of each track section at selected rates dependent upon traic condi tions, vehicle-carried apparatus comprising, receiver coils mounted in inductive relationship with'saidtrack rails, capacitance and associated damping resistance connected` with said coils to cause said coils to provide a distinctive output voltage of one polarity as each code pulse is applied to said rails and another distinctive output volt'- age of the opposite polarity as each codeV pulseis removed from said rails, a dual-triode type trigger circuit operable to either of two stable conditions and including Vtwo electron. tubes connected to said receiver coils, relay means operable to Vtwo different conditions and governed rby said trigger circuit so as to be operated to one of its conditions when said trigger circuit is'in one stable condition and to the other of its conditions when said trigger circuit is operated to its other stable condition, and decoding means governed by said relayy means and distinctively, operated according to thev coding rate-of the unidirectional pulses applied to the rails of said track circuits. i Y

10. A cab signalling system for railroads comprising, a plurality of coded track circuits having direct-current pulses applied to the track rails at different rates dependent upon traic conditions, vehicle-carried apparatus including receiver coils mounted in inductive relationship to said rails, capacitance shunting said receiver coils to form a tuned circuit for a relatively low frequency, resistance connected with said coils to dampen the oscillations of said tuned circuit, whereby a distinctive output voltage of one polarity is provided by each application of a code pulse to said rails and another distinctive voltage output is provided corresponding to each removal of a code pulse from said rails, code-following relay means operable between respective picked-up and dropped-away positions, means including electron tubes connected to said receiver coils for operating said relay alternately between said positions in response to said distinctive output voltages, and decoding means governed by said relay means and selectively operated according to the coding rate of the pulses applied to the track rails.

l1. In a cab signalling system for railroads, a plurality of track circuits and means for applying pulses of unidirectional current to said track circuits at selective rates according to tratc conditions, vehicle-carried equipment comprising, receiver coils positioned in inductive relationship to the track rails, capacitance`and resistance connected with said coils to respectively tune said coils and dampen the oscillations therein, whereby a distinctive voltage output of one polarity is produced at the beginning of each code pulse and another distinctive voltage output of the opposite polarity is produced at the end of each code pulse, amplifier circuit means including an electron tube biased above cutoff and having said distinctive output voltages applied between its grid and cathode thereby providing corresponding distinctive voltages across a load impedance included in the plate-cathode circuit of said electron tube, circuit means including two gaseous discharge tubes biased beyond cutoff, coupling circuit means for causing said distinctive voltages appearing across said loadimpedance to be simultaneously applied with opposite polarity between grid and cathode of each of said gaseous discharge tubes, relay means operable between respective'picked-up and dropped-away positions and including two windings, a plate-cathode circuit of each of said gaseous discharge tubes including a winding of said relay means, and decoding means selectively operated by said relay means according to the coding rate of the track current.

l2. A cab signalling system for railroads comprising, a plurality of track circuits and means for applying code pulses to the rails of said track circuits at selected D rates dependent upon traflic conditions, vehicle-carried apparatus including, receiver coils inductively positioned with respect to said rails, capacitance and resistance connected with said coils to respectively tune said coils and to dampen the oscillations appearing in said coils so that a distinctive voltage output of one polarity is provided corresponding to the beginning of each track code pulse and another distinctive voltage output of opposite polarity is provided corresponding to the end of each code pulse, amplifier means including an electron tube biased above cuto and having said distinctive voltages applied between its grid and cathode to produce corresponding distinctive voltages across a load impedance included in the plate-cathode circuit of said electron tube, circuit means including two gaseous discharge tubes biased beyond cutoff, coupling circuit means for causing said distinctive voltages appearing across said load impedance to be simultaneously applied with opposite polarity between grid and cathode of each of said gaseous discharge tubes, a condenser connected between the plate electrodes of said gaseous discharge tubes, relay Ineens including atwo-position relay operable top either of its direct-current potential, whereby the application of saidA distinctive voltage of positive polarity betweenl grid and cathode of one of said gaseous dischargejtubes causes said tube to become conductive and the resultant decrease of plate potential of said tube applied through said con-- denser to the plate of said other gaseous discharge tube contemporaneously with the application of said distinctive voltage of negative polarity between gridI and plate of said other tube causes said other tube to become nonconductive, thereby causing said relay to be actuated alternately to its diierent positions, and decoding means selectively operated by said relay means according to the coding rate of the track current.

i3. A cab signalling systetm for railroadsl comprising, a plurality'of track circuits and means for applying pulses or' unidirectional current to the track rails at selected rates according to tratiic conditions, vehicle-carried apparatus including receiver coils positioned in inductive relationship with said track rails, capacitance and resistance associated with said receiver coils to respectively tune said coils and to dampen the oscillations appearing therein, whereby transient output voltages are produced by said receiver coils at the beginning and end of each track code pulse, relay means including a two-position relay operable to either of its two different positions,'circuit means including 'electron tubes connected with said receiver coils and responsive to said transient output voltages for govemng the operation of said relay means alternately to one position and then the other, and decoding means selectively controlled by said relay means according to the coding rate of said track current.

14. In a cab signalling system for railroads comprising, a plurality of track circuits means located adjacent said track circuits for applying code pulses of unidirectional current to the track rails of said track circuits at selected rates in accordance with the existing traic conditions, vehicle-carried apparatus including receiver coils inductively associated with the track rails, capacitance connected with said receiver coils to tune them to a relatively low frequency, a resistance connected across,V said receiving coils to dampen the oscillations appearing therein whereby transient output voltages are produced by said receiver coils at the beginning and end of each track code pulse, circuit means including electron tubes connected to provide a trigger circuit operable to either of two stable conditions, said circuit means being connected to said receivers and responsive to said transient output voltages so as to be operated to its opposite stable conditions alternately, relay means including a two-position relay operable between its two diierent positions alternately by the operation of said circuit means, and decoding relay means selectively controlled by said relay means in accordance with its rate of operation as governed by the selected rate of the trackway code pulses.

l5. A cab signalling system for railroads comprising, a plurality of track circuits and means for applying pulses of unidirectional current to the rails of said track sections at different rates dependent upon traffic conditions, vehicle-carried equipment comprising receiver coils positioned in inductive relationship to said track rails, circuit means connected with said receiver coils for dampening the electrical oscillations in said coils caused by the application and removal of each track code pulse to give an output voltage of one polarity at the beginning of each code pulse and another output voltage of opposite polarity at the end of each code pulse, code following relay means including a two-position relay operable between its two different operated positions, amplier means including two electron tubes each having a control grid and cathode and plate, said control grid of each of such tubes connected manne 23 v24 to aterminal ofsaid receiver coils through a-grid resistor, saidielay means and selectively operated according to whereby'an'internttent Ashunting of said grid-and plateV the coding `rate of said track current. electrodes of veither of'said electron tubes produces a large References Cited .in the me of this patent voltage drop across the' associated grid resistor thereby preventing a large voltage drop vacross said receiver coils UNTED 'STATES PATENTS as might otherwise resultin transient oscillations appcar- 1,975,371 Place Oct. 2, 1934 ing across said receiver coils, circuit means connecting said 2,098,041 Hoppe Nov. 2, 1937 relay means to vsaid 'ai'nplifer means whereby said relay 2,197,417 Place Apr. 16, 1940 means is operated toits differentv positions in response to 2,297,119 Williamson et al. Sept. 29, 1942 said outputvoltages, Vand decoding means governed by 10 2,393,136 Bettisonl Jan. 15, 1946 

